The imaging of a point-type light source onto a detector is a general problem in optical metrology. If light of all colors is to have the most equal propagation times possible, the light path must run through only a limited distance in refractive material. This is particularly important if the process is carried out in the blue region or even in the near-UV region and a time window Δt≦100 fs (fs=10−15 s) is to be cut out across all colors simultaneously. Such arrangements are required in ultrafast spectroscopy. One typical example that may be mentioned is the so-called optical Kerr switch, which is used in time-resolved fluorescence spectroscopy and in Raman spectroscopy. Another example is the so-called “fluorescence upconversion” for the simultaneous observation of all relevant wavelengths.
The present invention relates in particular to all types of optical spectroscopy, including in conjunction with microscopy, which aim to achieve sub-picosecond time resolution of measurement light, be this spontaneously emitted light or transmitted light, preferably using an optical switch, wherein the latter is arranged between two crossed polarizers for the purpose of effective functioning. The cancellation ratio ε=T∥/T⊥ of the first polarizer to the subsequent polarizer (“analyzer”; T∥,⊥=transmission of the parallel or perpendicular setting) defines the signal-to-noise ratio (S/N), i.e. the sensitivity of the respective measurement.
The highest level of cancellation, εGLP>106, can be achieved with Glan polarizers (GLPs). These function as a result of birefringence in calcite crystals; their thickness is in principle approximately the same size as the width of the entry window, and the light beams may differ from the window normal in an acceptance angle of only up to ±2.5°. Glan polarizers are therefore used only in the parallelized beam path.
If a lot of light is to be collected, the parallelized beam diameter is naturally large, and so too then is the thickness of the calcite to be traversed, which limits time resolution and simultaneity. FIG. 2 analyses by how many picoseconds the light follows (lags behind) a reference pulse at 400 nm in the case of different wavelengths. For this it has been assumed that a 10 mm GLP is traversed. It can be seen that the spectral region from 320 nm to 450 nm which is of interest in biology is spread over a time window of 2.0 ps. For a time window of considerably <1 ps, therefore, the conventional type of GLP cannot be used.
The arrangements which are customary at present therefore use so-called wire grid polarizers (WGPs), the glass substrate of which may be less than 1 mm thick. However, the cancellation ratio thereof is only approximately εWGP≈103-104, three orders of magnitude below the cancellation that can be achieved with GLPs. The sensitivity of a polarization-dependent measurement is accordingly lower. For example, the S/N of time-resolved fluorescence spectroscopy using a Kerr switch is limited mainly by this shortcoming.
The observation of rapidly changing colors, be it during absorption or fluorescence measurements, is a frequently recurring measurement task. “Rapidly” in this connection means a time resolution of as far as possible less than 100 fs. In a fluorescence measurement (cf. L. Zhao et al., PCCP 7, 1716, 2005), a WGP, followed by a GLP, is used in order to suppress undesired background.